MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 143 AM EST SAT DEC 27 2014 VALID DEC 27/0000 UTC THRU DEC 30/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 00Z MODEL GUIDANCE ALONG WITH FINAL PREFERENCES/FORECAST CONFIDENCE... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 00Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...FULL-LATITUDE TROF MOVING ACROSS THE MIDDLE OF THE COUNTRY ON SATURDAY... ...ADVANCING COLD FRONT/LOW PRESSURE OVER THE GREAT LAKES... FINAL PREFERENCE: BLEND OF THE 00Z GFS/ECMWF FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE WATER VAPOR IMAGERY SHOWS A WELL DEFINED UPPER TROF ANCHORING THE FOUR CORNERS REGION WITH A LEAD IMPULSE CURRENTLY LIFTING THROUGH THE CENTRAL HIGH PLAINS. LIGHT TO MODERATE SNOW WAS NOTED FALLING ALONG THE ASSOCIATED SURFACE BOUNDARY STRETCHING FROM THE CENTRAL PLAINS UP INTO THE MIDDLE MS VALLEY/UPPER MIDWEST. THE INITIAL SHORTWAVE IS FORECAST TO PUSH INTO THE UPPER GREAT LAKES ON SATURDAY CARRYING THE DEVELOPING WAVE OF LOW PRESSURE ACROSS THE LAKES AND EVENTUALLY INTO SOUTHERN ONTARIO. MODELS SEEMS TO HAVE A DECENT GRASP ON THIS PATTERN WITH ONLY MINOR DIFFERENCES NOTED. A GENERAL MODEL COMPROMISE IS SUFFICIENT WITH THIS ASPECT OF THE SYSTEM. WHILE THE INITIAL SHORTWAVE ENERGY MOVES INTO THE GREAT LAKES...THE REMAINDER OF THE UPPER TROF WILL SLOWLY EJECT OUT OF THE FOUR CORNERS REGION. AN IMPRESSIVE SHORTWAVE WILL CROSS THE RIO GRANDE BY 28/1200Z WITH A LITTLE MORE SPREAD IN THE GUIDANCE THAN PREVIOUSLY SEEN. THE 00Z ECMWF/UKMET/CMC HAVE ALL TRENDED IN THE SLOWER DIRECTION...PARTICULARLY THE 00Z UKMET/CMC. THIS PLACES THESE MODELS ON THE SLOWER SIDE OF THE ENVELOPE OF SOLUTIONS. IN TIME...THE SHORTWAVE IS EXPECTED TO GRADUALLY SHEAR OUT AS IT RACES TOWARD THE MID-ATLANTIC BY LATE IN THE WEEKEND. STRONG SOUTHWESTERLY FLOW ALOFT WILL ALLOW THE FRONTAL ZONE TO HANG UP SOUTH OF THE MID-ATLANTIC TOWARD THE END OF THE PERIOD WITH MODEL DIFFERENCES NOTED. MOST NOTABLY...THE 00Z NAM IS MUCH SLOWER WHILE DEPICTING A SURFACE WAVE FARTHER NORTH THAN ANY MODEL. THE ACTUAL LOCATION OF THIS AREA OF LOW PRESSURE IS MORE UP IN THE AIR THAN PREVIOUSLY AS THE DETERMINISTIC SOLUTIONS HAVE SHIFTED FROM THEIR PRIOR RUNS. HOWEVER...THIS IS NOT TOO SURPRISING GIVEN THE VAST SPREAD NOTED IN THE ENSEMBLE LOW PLOTS. WILL STAY THE COURSE AND INCORPORATE THE NEW 00Z ECMWF INTO THE MIX. ...PRONOUNCED SHORTWAVE CROSSING THE PACIFIC NORTHWEST LATE SATURDAY... ...VORTICITY STRETCHING OUT OVER THE CENTRAL U.S... FINAL PREFERENCE: BLEND OF THE 00Z GFS OR 00Z GFS-PARALLEL WITH THE 00Z ECMWF FORECAST CONFIDENCE: AVERAGE WITH A SPRAWLING UPPER RIDGE SET UP ALONG THE EASTERN PACIFIC...A RATHER POTENT SHORTWAVE WILL BE ABLE TO DIVE FROM THE GULF OF AK DOWN TOWARD THE PACIFIC NORTHWEST EARLY IN THE FORECAST. MODELS AGREE THIS FEATURE WILL ENTER THE STATE OF WA BY 28/0000Z. AS THE HEIGHT FALLS SLIDE EASTWARD...THE VORTICITY SHOULD GRADUALLY BECOME MORE ELONGATED IN NATURE ACROSS THE GREAT PLAINS BY LATE SUNDAY/EARLY MONDAY. THE BIGGEST OUTLIER IS THE 00Z NAM WHICH IS SLOWER AND MORE AMPLIFIED OVER THE CENTRAL ROCKIES ON 29/1200Z. THEREAFTER...THE ENSEMBLE MEANS WHICH SEEM TO BE PLACED MORE IN THE MIDDLE GROUND SUGGEST THE 00Z CMC/UKMET ARE TOO QUICK TO EJECT THE ENERGY TOWARD THE MS RIVER VALLEY. THE DIFFERENCE IS NOT NEARLY AS LARGE AS THESE MODELS HAVE TRENDED IN THE SLOWER DIRECTION. THE RECOMMENDATION IS TO STAY CLOSER TO THE CENTER OF THE SPREAD WHICH MAKES A COMBINATION OF EITHER THE 00Z GFS/GFS-PARALLEL AND 00Z ECMWF REASONABLE HERE. ...ROBUST UPPER TROF DIVING INTO THE WESTERN U.S. EARLY NEXT WEEK... ...ASSOCIATED PLUNGE OF ARCTIC AIR... FINAL PREFERENCE: A NON-00Z UKMET MODEL COMPROMISE FORECAST CONFIDENCE: AVERAGE ON THE WINGS OF THE INITIAL SYSTEM CROSSING THE GREAT BASIN WILL BE A SIGNIFICANT SHOT OF COLD AIR TO THE WEST AS A LARGE UPPER TROF DROPS DOWN FROM FAR WESTERN CANADA. BY EARLY TUESDAY...A BROAD POSITIVELY-TILTED TROF WILL ANCHOR THE WEST WITH PRETTY REASONABLE AGREEMENT SUGGESTED ON THE 552-DM SPAGHETTI PLOT. THE EXCEPTION IS THE 00Z UKMET WHICH IS DECIDEDLY SLOW. THERE IS A TREND IN THE FASTER DIRECTION WHICH MAKES THE 00Z ECMWF LOOK MORE LIKE THE LAST TWO RUNS OF THE UKMET. IT APPEARS THE MODELS ARE STILL ADJUSTING SO WILL LOWER THE CONFIDENCE TO AVERAGE AND UTILIZE A NON-00Z CMC MODEL COMPROMISE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER